1. Field of the Invention
The present invention relates to an electrochemical device.
2. Related Background Art
As portable electronic devices have been becoming widespread, there have been increasing demands for lightweight, small-size electrochemical devices which can continuously be driven for a long time, such as secondary batteries. While conventional secondary batteries used metal outer cans, it has become possible to reduce the battery weight by employing thin, lightweight films for outer bags, as typified by lithium polymer batteries, thereby increasing the degree of freedom in designing.
When some abnormalities happen in a battery using such a film as its outer bag, gases may be generated or, in the worst case, ignition may occur depending on kinds of electrolytes in use. While chargers are set such as to stop charging when reaching a predetermined time or voltage, electrochemical devices will be overcharged if the charging does not stop before exceeding their capacity by some reason. As the overcharged state further progresses, electrolytes may decompose, so as to yield gases, which may inflate outer bags and cause internal short circuits due to internal deformations, thereby exploding the bags or igniting the electrochemical devices,
It has also been known that secondary batteries using lithium ions are likely to cause thermal runaway when a certain temperature is exceeded. Thermal runaway generates gases and further heat, thereby exploding or igniting the batteries.
For evading such circumstances, mechanisms provided with safety valves so as to release gases when the internal pressure rises have been under study, for example, as described in Japanese Patent Application Laid-Open Nos. 2000-100399 and HEI 11-312506.